This invention relates to sealable and sterilizable packages made of metal foil or light-gauge sheet suitable for containing products such as foodstuffs and the like. More particularly, it relates to packages of the type described including a bottom and a lid readily separable by manual peeling of the lid for access to the contents. In an important specific sense, the invention is directed to aluminum foil and/or light gauge sheet packages having a bottom member and lid member each internally lined with a coating of thermoplastic material. The term "aluminum," as herein used, embraces aluminum metal and alloys thereof.
For simplicity of discussion, detailed reference will be made herein to aluminum foil containers as an illustrative and indeed especially advantageous example of the types of containers with which the present invention is concerned; but it is to be understood that in a broad sense the invention generally embraces containers made of metal foil and/or light-gauge metal sheet.
Aluminum foil and light-gauge sheet are widely employed in the packaging of foodstuffs, e.g. in containers for retail sale. By way of example, an aluminum foil package for such purposes may comprise a relatively stiff, shaped foil bottom and a lid of foil lighter in gauge than the bottom, with an internal thermoplastic lining or coating on both the lid and the bottom to separate the contained food from the metal of the foil. These linings must be inert with respect to the foodstuff; i.e. they must not exhibit water staining or blushing or affect the taste of the food.
Aluminum foil containers and the like have heretofore commonly been used for packaging frozen foods. There are various disadvantages associated with frozen foods, however, including the necessity of providing refrigerated storage as well as the energy consumption and sometimes inconvenient delays involved in heating the food from frozen condition before or during cooking. It has, of course, long been known to package foods for preservation at room temperature under sterile conditions by canning, but metal containers for that purpose have conventionally been relatively heavy cans adapted to be opened only by means of a tool.
The provision of a manually openable aluminum foil or like container, which can be sealed and sterilized to provide shelf-stable packaging of foods at room temperature (i.e. packaging that preserves the contained food in unimpaired and safely usable condition for extended periods of room-temperature shelf storage) constitutes a particularly important aspect of the present invention. Such packaging is desirable as obviating both the disadvantages of frozen foods and the inconvenience and other drawbacks of conventional cans. In this aspect, the invention is concerned with provision of a foil or like container that is capable of replacing conventional "tin cans" for shelf-stable room-temperature food storage, as distinguished from frozen food packages, and which provides at the same time the advantages of light-weight and easy manual access to the contents without use of a can opener.
For these and other uses of aluminum foil containers, preservation of the contents in satisfactory condition for safe use requires that the container be sealed and sterilized. It has heretofore been proposed to seal the lid and bottom thermoplastic linings of an aluminum foil package together, utilizing as the lining materials thermoplastic substances which are stable at sterilization temperatures. A typical sterilization temperature for many food products is about 120.degree. C.
At the same time, it is desirable that such a package be readily manually openable for convenience of access to the contents by a consumer. Prior foil packages adapted for sealing and sterilization have been difficult to open, either tearing unevenly when an attempt is made to separate the lid from the bottom, or requiring the use of an implement such as a knife for cutting through the lid and its associated lining. Efforts previously made to achieve a sealable and sterilizable shelf-stable foil package in which the lid and bottom are easily separable by parting of the seal between them, so as to enable manual peeling of the lid from the bottom, have been unsatisfactory owing to failure to achieve a reliably sealed and sterile package. This failure is presently believed attributable to the fact that the lid lining material selected to provide a seal separable by peeling has failed to maintain satisfactory adherence to the foil surface of the lid and/or to the lining material of the bottom with which it is supposed to form a seal.
A further important consideration, in a practical sense, is that opening of the package should not result in visible and unsightly irregularities along the rim of the bottom. Such irregularities are aesthetically undesirable, especially where the bottom may be used as a serving vessel for the contents, and may seriously detract from consumer acceptance of the package.